neutron capture and waste transmutation - uvific.uv.es/~euschool/lec_tain.pdf · j.l. tain neutron...

Neutron Capture and WasteTransmutation

J.L. Tain

Instituto de Física Corpuscular

C.S.I.C - Univ. Valencia

Euro Summer School on Exotic Beams Valencia, September 4-12, 2003

J.L. Tain Neutron Capture and Waste Transmutation 2

Layout of the lectures

• Transmutation of radioactive waste, ADS& Nuclear Data (F J. Benlliure)

• Neutron capture: theory and practice

Nuclear Waste, Transmutation,ADS

& Nuclear Data


Nuclear power plants in the world:

• 441 plants in operation,32 under construction

http://www.iaea.org

UE Total: 141


Nuclear electricity generation:

Total:

• Installed: 359 GW(e)

• Produced: 2574 TWh(81.7% availability)

Share:

• World average: 17.3%

• EU average: 33.6%


Nuclear Fission Reactors

Most reactors in use are ofLight Water Reactor type, eitherBoiling Water Reactors orPressurized Water Reactors:

Fuel: UO2 , 2- 4% 235U

Moderator-Coolant: H2O

n-spectrum: thermal

PWRn

235U

fissionfragments

n

n

235U

CHAIN REACTION

Reactors operate at CRITICAL POINT(neutron balance) which must be kept:REACTOR CONTROL


Nuclear waste generation:

The loaded fuel is transformed during the energy generation process:

n + 235U →→ 3n + 134I + 99Y;

134I(52min) →→ 134Xe.

99Y(1.5s) →→ 99Zr(2s) →→ 99Nb(15s) →→ 99Mo(66h) →→ 99Tc(2.1×105y)

n + 238U →→ 239U;

239U(23.5min) →→ 239Np(2.4d) →→ 239Pu(2.4×104y)

Actually a complex set of reactions will take place…

FFission ission PProductroduct

TRTRans-ans-UUranicsranics

n + 239Pu →→ 240Pu(6.5 ×103y); n + 240Pu →→ 241Pu(14.4y) →→ 241Am(432y)

Minor Actinides

FISSION

CAPTURE


Partial reaction chain of the U-Pu cycle:


As a consequence an inventory of long lived highly radioactiveisotopes builds up in a reactor ∀ High Level Waste

266 kg Pu (156kg 239Pu)

20 kg MA

946 kg FP ( 63kg long-lived FP)

0,062%

1,003%

0,097% 0,002%

0%

1%

2%

3%

4%

5%

Fragmentosde Fisión

Neptunio Plutonio Americio Curio

A 1GWe (3GWth) Pressurized Water Reactor, producing 7TWh (80% avail.),burns 1 ton/year fissile material.

Loaded with 27.3 Ton of 3.5% enriched UO2 (954kg 235U) produces after aburn-up of 33GWd/ton (~1 year):

and still contain 280kg 235U plus111kg 236U


The hazard they represent can be measured by the evolution oftheir radio-toxicity:


One possible strategy to reduce the hazard of HLW in the long term isprovided by transmutation (and/or incineration):

Actinides:

• Fission:

n + 243Am(7.4×103y) →→ FF’s + n’s

• Capture (+ Decay) + Fission:

n + 243Am →→ 244Am(10.1h) →→ 244Cm;

n + 244Cm(18.1y) →→ FF’s + n’s

Long-lived FP:

• Capture:

n + 99Tc(2.1×105y) →→ 100Tc(15.8s) →→ 100Ru.

Is it possible to usethe same reactionsthat create the wasteto destroy it?

The key point is theseparation of thedifferent componentsfrom spent fuel (orpartitioning)


Neutron induced reactions: strong energy dependence …

235U


…and isotope dependence:

Fission: Capture:

Therefore there are several possible (from physics point of view)solutions to the problem of “burning” HLW


For example, the burning of TRU isfavoured by “fast” n spectrum ...

In any case asurplus of nare needed totransmute …


The scientific considerations,together with technological,military and politicalconsiderations has lead to theproposal of different schemesfor the management of HLW.

A currently consideredscheme is the doublestrata scenario: FF


Accelerator Driven Systems:

A nuclear reactor with a subcritical core which uses an accelerator toproduce the neutrons necessary to maintain the chain reaction:

For example: a veryhigh intensity ~1 GeVproton acceleratorproducing neutronsby spallation on amolten Pb or Pb/Bitarget which acts alsoas the coolant, withsolid actinide fuel.

Advantage: lessdependence on fuelcomposition


An ADS can also be utilized for energy production and if based onTh-U fuel cycle, will generate a reduced amount of TRU ...

ThU-cycle

UPu-cycle

fertile

fissile

Energy Amplifier


Present in nuclear wastes Thermal and Fast FissionMedium Half-Life (<100 años) Fast FissiónShort Half-Life (< 30 dias) Low Fission Cross SectionHigh A actinides

Av. Flux Intensity (n/cm2/s)3,00E+15

Cm242 Cm243 Cm244 Cm245 Cm246 Cm247Second 1 Time Unit α α α / SF α / SF α / SF αHour 3600 31570560 100 / 6.2E-6 9 9 . 7 / 0 . 2 9 / 5 . 3 E - 9 100 / 1.35E-4 100 / 6.1E-7 100 / 3E-2 100

Day 86400 0,446 29,068 18,080 8490,695 4724,813 15582935,494

Year 3E+07 18,130 2,798 6,257 2,922 16,459

64,7% 8,0% 65,2% 11,4% 44,6%

Am241 Am242 Am242m Am243 Am244α / SF β− / EC IT / α / SF α / SF β− / EC

100 / 3.77E-10 82.7 / 17.3 9 9 . 5 / 0 . 4 6 / 1 E - 3 100 / 3.7E-9 100 / 4E-2

432,225 0,002 140,846 7361,922 0,001

3,652 17,792 1,844 4,892

44% : 44% 13,1% 8,4% 87,0%

Pu238 Pu239 Pu240 Pu241 Pu242 Pu243α / SF α / SF α / SF β− / α α / SF β−

100 / 1.9E-7 100 / 3.1E-10 100 / 5.7E-6 100 / 2.45E-3 100 / 5.5E-4 100

87,644 24083,608 6556,805 14,334 372891,707 0,001

4,220 3,477 9,033 2,688 11,354 6,775

37,5% 19,4% 54,8% 14,2% 61,1% 30,6%

Np237 Np238 Np239 Pu239 Symbol & Massα / SF β− β− α / SF Decay modes

100 / 2E-12 100 100 100 / 3.1E-10 Branching ratios2137656,095 0,006 0,006 24083,608 Half-Life

4,332 15,928 3,477 Absorption-Half-Life81,5% 13,1% 19,4% (n,γ)/absoption

TRU Transmutation SchemeFast Spectrum

Ln(2)/(σφ)

Transmutation of TRU: set of reactions


Transmutation of TRU: new fuel compositions


Challenges in the field of ADS:

• Waste separation (or partitioning) methods

• Design of a high power accelerator, the spallation targetand their coupling

• Study of reactor core behaviour and transmutation rates

Need for new or improved accuracy nuclear data:

• proton spallation reaction: n yield, residues (FF J. Benlliure)

• neutron induced reactions: fission, capture, (n,xn),… onactinides, fission products and structural materials


An example of poorly known reaction…


…another example of not so poorly known

Neutron radiative capture:Theory and practice


Neutron reactions at low energies

A neutron is absorbed to form a “compoundnucleus”:

n + AZ →→ A+1Z*

which lives for a short time and decays:

A+1Z* →→ n + AZ (elasticelastic)

A+1Z* →→ n + AZ* (inelasticinelastic)

A+1Z* →→ A+1Z*’ + γγ (radiativeradiative capture capture)

A+1Z* →→ A1Z1* + A2Z2* + xn (fissionfission)

A+1Z* →→ A+1-xZ* + xn (n multiplicationn multiplication)

…

Other contributions:


The CN formation probability is higher for certain neutron energies Encorresponding to quasi-bound or virtual states: resonances

ER = Sn + En Sn : neutron separation energy of CN ( <10MeV )→→ level separation D0 ~ 1 eV – 100 keV

A

A+1

ΓΓ = ΓΓn + ΓΓγγ + ΓΓf + …

(σσ = σσn + σσγγ + σσf + ...)

Life-time ↔↔ Energy-width: ΓΓ

ΓΓ ~ 1 meV – 100 keV


log En

1/v resolved unresolved overlapping

log

σσ

resolution

ΓΓ > D0 : overlapping resonances

ΓΓ < D0, ΓΓ > ∆∆E: resolvedresonance region (RRR)

ΓΓ < D0, ΓΓ < ∆∆E: unresolvedresonance region (URR)

1/v: thermal

Shape of neutron cross-section

• In the RRR region, σσ is described using the R-Matrix formalism, inone of its usual approximations.

• In the URR region, average σσ are described by Hauser-Feshbachstatistical theory

• It is a parametric approach since nuclear theory cannot predict thevalues.

• Experimental information is strictly necessary.


Single Level Breit-Wigner Formalism: (n,γγ)

σσγγ(E) = ππDD2 gJ

ΓΓn ΓΓγγ

(E-ER’)2 + ¼ΓΓ2

For ll-capture into an isolated spin J resonance at ER:

DD = hh/ √√ 2µµE (neutron wavelength)

gJ =2J + 1

2(2I + 1)

(spin stat. factor;

|I - ll ± ½| ≤≤ J ≤≤ |I - ll + ½| )

ΓΓ(E) = ΓΓn(E) + ΓγΓγ + …; (FWHM)

ΓΓn(E) = √√ E/ER ΓΓn(ER) , ll=0

… and the channel radius Rc

n+197Au, ll=0, J=2

ER = 4.9 eV

ΓΓn = 15.2 meV

ΓΓγγ = 122.5 meV

En (eV)

σσ γγ (b

arn

)


SLBW formalism: elastic and capture cross sections

P0 = ΦΦ0 = k RC = ρρ, S0 = 0

Pll = ρρ2 Pll-1 / ((ll- Sll-1 )2 + Pll-12)

Sll = ρρ2(ll-Sll-1) / ((ll- Sll-1 )2 + Pll-12) - ll

ΦΦll = ΦΦll-1 – tan(Pll-1 / (ll -Sll-1))

ER’ = ER + ΓΓn(ER)Sll(ER) - Sll(E)

2Pll(ER)

ξξ = (E-ER’)2

ΓΓΓΓn = ΓΓn(ER)

Pll(E)

Pll(ER)

ΓΓ

ΓΓγγ ΓΓn

1 + ξξ2

1σσγγ = gJ4ππ

k2k = √√2µµE / hh

σσ = gJ [ sin2 ΦΦll + cos 2ΦΦll + sin 2ΦΦll ]ΓΓn

ΓΓ 1 + ξξ2

1 ΓΓn

ΓΓ

ξξ

1 + ξξ2

4ππ

k2

gJ =2J + 1

2(2I + 1)

ELASTICCAPTURE

potential resonant interference

(n,γγ)

(n,n)

238U


• From the analysis of experimental data on capture, total, fission, …cross-sections, resonance parameters are obtained for every nucleus.

• All the information is combined, cross-checked for consistency, etc, ina process called “evaluation” until a recommended set of parameters isobtained.

• This information is published in a Evaluated Nuclear Data File using anaccepted standard format (ENDF-6)

• There exist several files:

• BROND-2.2 (1993, Russia)

• CENDL-3 (2002, China)

• ENDF/B-VI.8 (2002, US)

• JEFF-3.0 (2002, NEA+EU)

• JENDL-3.3 (2002, Japan)

Neutron Reaction Data


Measurement of (n,γγ) cross-sections

σσγγ(E) =Number of capture reactions

Number of target nucleus per unit area x Number of neutrons of energy E

Nγγ

nT [at/barn]·Nn(E)σσγγ(E) = Nn nT Nγγ

Needs:

• sample of known mass and dimensions

• count the number of incident neutronsof energy E

• count the number of capture reactions

… but there are anumber ofexperimentalcomplications


Neutron Beams

• Need to span a huge energy range: 1meV – 100MeV

• Since neutrons cannot be accelerated, they have to be producedby nuclear reactions at certain energy and eventually deceleratedby nuclear collisions (moderated)

• Energy determination:

• kinematics of two-body reaction

• mechanical selection of velocities (“chopper”)

• Time Of Flight measurement

• Sources:

• Radioactive

• Nuclear detonations

• Reactor

• Light-ion accelerator

• Electron LINACS

• Spallation

En = ½ mnL2

t2


ForschungsZentrum Karlsruhe Van de Graaf

• 7Li(p,n)7Be, Q= -1.644MeV

• Ep ~ 2MeV →→ En ~ 5-200keV

• Rate: 250kHz, ∆∆t = 0.7ns

30keV > Thresh.

7Li-target


Geel Electron LINear Accelerator

• (γγ,n), (γγ,f) on U (bremsstrahlung)

• Ee ~ 100MeV, Ie ~ 10-100µµA

• Rate: 100-800Hz, ∆∆t ~ 0.6-15ns


… GELINA

U-TARGET &H2O MODER.

NEUTRON SPECTRA

H2O MODERATOR

n yield vs. Ee

1 MeV

1 eV

H2O Moder.

Bare U-target

Polyethylene Moder.


CERN neutron Time Of Flight

n_TOF

• p-spallation on Pb target

• Ep = 20GeV, Ip = 7x1012 ppp

• Rate: 2.4s-1 , ∆∆t = 14ns


PROTON BEAM LINE

p current monitor

p-intensitypickup

Entrance tothe target

Water cooledPb target

n beam tube

shielding

NEUTRON BEAM LINE

shielding

n_TOF


Bending magnet

2nd collimator

shielding

EXPERIMENTAL AREA

n monitor

Sample changer

BEAM DUMP

n monitor

… n_TOF


• The characteristics of the spallation-moderation process and thecollimators in use determine the neutron beam parameters: intensity-energy distribution, energy resolution and spatial distribution.

TARGET:

80x80x60 cm3 Pb + 5cm H2O

SPALLATION PROCESS: ~600 n/p

MODERATION:

ξ = ⟨⟨ ln(E i/Ef) ⟩⟩ ≅≅ 1+ ln

( ξξ(H)=1, ξξ(Pb)=0.01 )

A-1

A+1

(A-1)2

2A

Intensity distribution

… n_TOF


The statistical nature of the moderation process produces variationson the time that a neutron of a given energy exits the target assembly

ResolutionFunction

… also important:time spread of beam

RF

∆∆t beam

Beam energy-resolution

time vs. energy

RF @ 5eV

RF @ 180keV

∆∆t2

t2

∆∆L2

L2

∆∆En2

En2

= 2 + 2

… n_TOF


The collimation system determines the final number of neutronsarriving to the sample an its spatial distribution

Neutrons on sample

10-5

Beam profile

4cm ∅∅

… n_TOF


Neutron Intensity Monitoring:• Reaction: n + 6Li →→ t + αα

• Si detectors

Si

Si

Si

Si 200µµg/cm2 on3µµm Mylar

… n_TOF


Techniques for radiative capture detection:

• Detection of the capture nucleus

• Activation measurements

• Detection of γγ-ray cascade

• Total Absorption Spectrometers

• Total Energy Detectors

• Moxon-Rae Detectors

• Pulse Height Weighting Technique

n

• Irradiation: A(n,γγ)A+1

• A+1 radioactive with suitable T1/2

• Measurement of characteristicγγ-ray of known I γγ with Ge detector


εεγγi : total efficiency for γγ-ray of energy Eγγi

εεpγγi : peak efficiency for γγ-ray of energy Eγγi

Define:

Then:

εεC = 1 - ΠΠ (1 - εεγγi)i=1

mγγ

total efficiency for cascade:

εεpC = ΠΠ εεp

γγii=1

mγγ

peak efficiency for cascade:

If εεpγγi = 1, ∀∀i ⇒⇒ εεp

C = εεC = 1 Total Absorption Spectrometer

Total Energy Detector

If εεγγi « 1 & εεγγi = kEγγi , ∀∀i ⇒⇒ εεC ≅≅ ΣΣ εεγγi = k ΣΣ Eγγi = k ECi=1

mγγ

i=1

mγγ

Eγγ1

Eγγ2

Eγγ3

EC


n_TOF Total Absorption Calorimeter

• 40 BaF2 crystals

• ∆Ω∆Ω/4ππ = 95%

• ∆∆E ≅≅ 6%


(from Karlsruhe 4ππ BaF2 detector)

BaF2(n,γγ) contamination

εε = 95% BACKGROUND REDUCTION

Deposited energy


245Cm(n,γγ)

Deposited Energy

245Cm(n,γγ)

Multiplicity

• The good energy resolution and detector granularity makesfeasble the measurement of fisioning nuclei

n_TOF TAC

(Monte Carlo simulation D. Cano - CIEMAT)


Total Energy Detectors

• Moxon-Rae type detectors:

The proportionality between efficiency and γγ-ray energyis obtained by construction:

(γγ,e-) converter + thin scintillator + photomultiplier

γγ e-

Maximumdepth ofescapingelectronsincreaseswith Eγγ …

But … proportionality onlyapproximate (need corrections)

Not much in use nowadays

Bi-converter

C-converter

Mo-converter

Bi/C-converter

εε γγ /

Eγγ

Eγγ


• Pulse Height Weighting Technique:

Total Energy Detectors

The proportionality between efficiency and γ-ray energy is obtained bysoftware manipulation of the detector response (Maier-Leibniz):

If Rij represents the response distribution fora γ-ray of energy Eγj:

ΣΣ Rij = εε γγji=1

imax

ΣΣ Wi Rij = Eγγji=1

imax

it is possible to find a set of weighting factorsWi (dependent on energy deposited i) whichfulfil the proportionality condition (setting k=1):

for every Eγj

Rij

E γγ = 9MeV

E γγ = 2MeV

Wi


How accurate can be theweighting function?

Using GEANT Monte Carlogenerated responses with fulldescription of setup (includingsample): σσσγσγ ≤≤ 2%

Fix “experimental” WF

sampledependent←←

1.15keV @56Fe

1.15keV @56Fe


Detectors: C6D6 liquid scintillators

Advantage: low neutron sensitivity

Also detector dead material isimportant …

OptimizedBICRON

HOME MADE:

• C-fibre cell

• No cell window

• No PMT housing

… and surrounding materials

εεγγ = 3-5%


Acquiring the data: full train of detector pulses

• Digitizer: 8bit-500MS/s FADC + 8MB memory

• On-line “zero” suppression

TOF & EC6D6

from pulse-shape fit

of PMT anode signal

• ∆∆t = 2ns

• En down to 0.6eV

• 0.5 MB/pulse/detector


++ Yield: Yγγ (E) = Nγγ/Nn(E)

Nγγ

nT ·Nn(E)σσγγ(E) =

–nT ·σσ(E)• Self-shielding: Yγγ (E) = (1 - e )

σσγγ(E)

σσ(E)

• Multiple scattering correction:elastic collision(-s) + capture

• Thermal (-Doppler) broadening

Sample effects:

Beam effects:

Analysing the data

• Resolution Function

shielded

Single-scattering

T = 300K

58Ni(n,γγ)

ER =136.6keV


Use a R-Matrix code as SAMMY to fit the data and extract the parameters:ER, ΓΓγγ , ΓΓn, …

197Au(n,γγ)

ER= 4.9eV

56Fe(n,γγ)

ER= 1.15keV

RF

T broad. single-scattering

doublescattering

Y =ΓΓγγ

ΓΓ


Slow neutron capture nucleosynthesis (s-process)

• Pb-Bi isotopes: termination-point of s-process & ADS target-coolant


800 eV

2.3keV

3 keV4.4keV

5 keV

12 keV

20 keV

209Bi(n,γ)

208Pb (n,γ)

354keV

527keV

17820 eV

2310 eV

5112 eV

12098 eV

15510 eV

802 eV

l = 0

9760 eV

4456 eV

6286 eV

6524 eV

9708 eV

3348 eV

l = 1

17440 eV17820 eV

20860 eV

21050 eV

209Bi(n,γγ)Preliminary dataon Pb-Bi capture

n_TOF σσγγlower for ll=0


Bibliography:

• Nuclear Engineering, R.A. Knief, Taylor & Francis Ltd., 1992

• Hybrid Nuclear Reactors, H. Nifenecker et al., Prog. Part. Nucl.Phys. 43 (1999) 683

• http://www.radwaste.org

• The Elements of Nuclear Interaction Theory, A. Foderaro, MIT Press,1971

• Neutron Sources for Basic Physics and Applications, Ed. S.Cierjacks, NEA/OECD, Pergamon Press, 1983

• Neutron Radiative Capture, Ed. R.E. Chrien et al., NEA/OECD,Pergamon Press, 1984

• Evaluation and Analysis of Nuclear Resonance Data, F.H. Froehner,NEA-OECD, JEFF Report 18, 2000

neutron capture and waste transmutation - uvific.uv.es/~euschool/lec_tain.pdf · j.l. tain neutron...

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